Outdoor combustion air intake furnace problem

[ch35iM]

Bought a new house with a Ruud Achiever 90 90+ efficiency gas furnace in a basement laundry room. It has PVC intake and exhaust pipes that run about 30 feet to the outside.

There are 2 cutouts for intake piping on the furnace, one is where the current intake pipe is sealed up to on top and the other is directly next to it but on the side, an alternate intake configuration if the unit was installed horizontally. There is a rubber cap for whichever intake port isn’t being used. Over the summer I noticed this cap on the floor next to the furnace. Assuming it fell out or something I popped it back in the alternate intake cutout and forgot about it.

Fast forward to now with the weather cooling off I had to turn on the heat, everything is seemingly working fine from upstairs but earlier today when doing laundry down there the furnace kicked on and I noticed a quite scary “click-suck-click-suck-click-suck-click-burners light.”

The “suck” part is best described as a vacuum created within the furnace. The front service door gets sucked inward quickly and then released. I replicated the problem on lighting a few times and the pattern is the same: 4 “click-sucks”, and then the burners light and everything is normal. Figuring there was an issue with the intake I took the cap off the alternate intake on the side and sure enough the furnace when kicking on clicks once, draws air through the hole, and lights without a hitch.

I inspected the intake pipe for clogs but it appears to be fine visually from both ends. I didn’t snake it yet or blow air through it to see if it isn’t clogged in the middle somewhere but before I did that I wanted to see what could be going on here.

Is it definitely lack of air flow though the intake pipe? Should I just leave the cap off the alternate intake hole and let it suck in air from Inside? I wonder how long the previous owner (original owner, 10 year old house, original furnace) had the cap off and was it done purposely. I still assume I’m correct that the cap should be plugging the cutout and all air should come in through the outdoor intake pipe but some guidance would be appreciated if that isn’t correct.

[pacnw]

Needs to be inspected by a competent tech!!!